1. Field of the Invention
The present invention relates to an organic oxidation catalyst to oxidize an alcohol, particularly an organic catalyst excellent in environmental harmonization and its preparation process. The present invention particularly relates to a technique to selectively oxidize an alcohol to an aldehyde, a ketone and/or a carboxylic acid, based on a nitroxyl radical.
2. Discussion of Background
Oxidation of an alcohol to a carbonyl compound is one of the most basic reactions in organic synthesis and heretofore, many excellent oxidizing agents and oxidation methods have been developed. Heretofore, oxidation reaction of an alcohol has been conducted by an oxidizing agent using a heavy metal such as a transition metal. However, a heavy metal such as a transition metal adversely affects the environment, and due to importance of the oxidation reaction of an alcohol, further increase in efficiency and improvement of the environmental harmonization are desired for the oxidation reaction of an alcohol.
In recent years, instead of a conventional oxidizing agent using a heavy metal, 2,2,6,6-tetramethylpiperidine N-oxyl (hereinafter sometimes referred to as “TEMPO”) has been widely used as an oxidation catalyst for an alcohol since an experiment was disclosed in Non-Patent Document 1. The reaction mechanism is shown in FIG. 1. TEMPO is considered to be a low environmental burden type organic oxidation catalyst as compared with a heavy metal, and a combination of various bulk oxidizing agents with TEMPO, such as low environmental burden type NaOCl and PhI(OAc)2, has been attempted as disclosed in Non-Patent Documents 2 and 3.
However, even TEMPO which has been widely used as an oxidation catalyst for an alcohol still has several problems. TEMPO is an excellent primary selective oxidation catalyst for a substrate in which a primary hydroxyl group and a secondary hydroxyl group coexist, but is not effective for oxidation of a secondary alcohol having a more sterically complicated structure and has limits to the catalytic activity. Further, TEMPO has a problem in stability in chemical structure such that decomposition is likely to occur as shown in FIG. 2 due to its chemical structure. Very recently, as an environmentally harmonizing oxidation catalyst which solves the above problems, 1-methyl-2-azaadamantane N-oxyl (hereinafter sometimes referred to as a “1-methyl-AZADO”) which is another adamantane type nitroxyl radical as shown in FIG. 3 has been found by the present inventors. 1-Methyl-AZADO is an organic oxidation catalyst which applies a slight burden to environment like TEMPO, and which not only provides a higher catalytic turnover to a primary alcohol than TEMPO but also is capable of oxidizing a secondary alcohol having a sterically complicated structure, which is hardly oxidized by TEMPO, with high efficiency (Non-Patent Document 4).
As described above, 1-methyl-AZADO exhibits high catalytic activity not only to a primary alcohol but also to a secondary alcohol. However, its preparation is economically problematic since six steps are required from a commercially available compound, and use of expensive reagents is inevitable.
9-Azabicyclo[3.3.1]nonane N-oxyl (hereafter sometimes referred to as “ABNO”) which is a bicyclo type nitroxyl radical highly structurally homologous to 1-methyl-AZADO having an adamantane skeleton, is a compound ranking as one of structural analogs of an azaadamantane type nitroxyl radical. Since it has been found to be present as a stable free radical like TEMPO, researches on its physical and chemical properties and researches on its application (e.g. a radical trapping agent, a radical generator, a spin labeling agent, etc.) as well have been conducted (Non-Patent Documents 5 and 6). However, its utilization as an alcohol oxidation catalyst has not been conducted at all.
ABNO is a known compound, but known preparation processes (including a process for preparing an amine-form as a precursor) have not been satisfactory from the viewpoint of mass supply (Non-Patent Documents 7 and 8).    Non-Patent Document 1: Golubev V. A. et. al: Izv. Akad. Nauk SSSR, Ser. Khim. 1965, p. 1927    Non-Patent Document 2: Lidia D. L., et al: J. Org. Chem. 2003, vol. 68, p, 4999    Non-Patent Document 3: Miller R. A., et al: Org. Lett. 2003, vol. 53, p 285    Non-Patent Document 4: Shibuya, M., et al: J. Am. Chem. Soc. 2006, vol. 128, p. 8412    Non-Patent Document 5: Mendenhall G. D., et al: J. Am. Chem. Soc. 1973, vol. 95, p. 6395    Non-Patent Document 6: Engel P. S., et al. J. Am. Chem. Soc. 2001, vol. 123, p. 3706    Non-Patent Document 7: Nelsen S. F., et al: J. Am. Chem. Soc. 1978, vol. 100, p. 7876    Non-Patent Document 8: Momose T., et al: J. Chem. Soc., Perkin Trans., 1, 1997, p. 1307